There is no admission that the background art disclosed in this section legally constitutes prior art.
There are many different kinds and types of battery powered devices and apparatus. For example, battery powered vehicles are coming into increasing use. New battery technologies such as Lithium Ion are playing an increasing role in that application. One of the most important considerations for them is their range. While Lithium Ion cells provide substantial advantages over the old lead acid technology, such as larger capacity to weight and capacity to volume ratios, they have also presented certain challenges to realizing their full potential. In an application such as electrically powered vehicles, it is necessary to connect cells in series in order to achieve practical voltage levels to be compatible with available controllers and motors.
Doing so presents an issue relating to battery cell balance. If one cell in a series string charges significantly differently from the others, it can cause performance to be degraded. For example, many cell strings or battery packs use a Battery Management System (BMS) to monitor battery conditions. Since cells are normally charged in series by one charger, a single cell in a string having a voltage that is too high during the charge cycle may trigger the BMS to shut down the charger. If that high cell had a voltage much higher than the others in the battery packs, the others could have had their charge cycle terminated prematurely in order to protect the high cell. Thus, the other cells may not be fully charged, thereby resulting in the vehicle range being undesirably reduced.
Conversely, during discharge, a cell that has a voltage much lower than the others could trigger the BMS to initiate vehicle shut down, even though the other cells may have had reserve capacity, once again reducing the range undesirably. It can be seen that both unbalanced high, and unbalanced low cells may cause the previously described range problems.
A charge imbalance can occur from several sources. For example, different cells in series may store energy at different rates even though they all share the same charge current. Also, some cells in use may have more discharge current than others if, for example, they comprise a portion of the cells that are sometimes used to power a low voltage load such as lights. Thus, it may be desirable for some applications to provide substantial charge balance in a series string of cells, to improve the amount of useable energy stored in them. In a vehicle application, this may provide the delivery of an improved range for given applications.
In the past, several approaches have been taken in an attempt to accomplish balance in a series string of cells. A common approach has been to compare voltages of the different cells during charge, and shunt some current around the higher voltage cells to reduce their charge current while leaving the full current flowing through the lower voltage cells. This approach may work for some chemistry batteries, but not well for Li-Ion chemistry cells. One reason is that for some applications, it may well be difficult or impossible to determine the amount of the state of charge on individual cells during the charging process.
This limitation can be seen by referring to FIG. 1. Here, the voltage is shown as a function of energy flowing into a partially discharged Li-Ion cell. It can be seen that the voltage is almost constant during most of the charging process, and only changes near the end of charge. It is therefore difficult to determine the state of charge by looking at the cell voltage until it is almost completely charged. Because of this, cells with differing states of charge may not be accurately differentiated from one another during most of the charge process, and therefore appropriate current shunting may not readily be accomplished at least for some applications. When voltage differences do become evident, the charge is nearly complete for the highest cells, and it may be too late to accomplish a balance by making a modest reduction in the current through them, since the conventional BMS may turn off all charging when the first cell may be fully charged and its voltage may reach the maximum allowable. At that point, the lower voltage cells may not be fully charged, and thus, the imbalance may remain.